1. Technical Field
The present invention relates to a power generation unit for taking out the charge, which is generated when a piezoelectric material such as of a piezoelectric element is deformed, as electrical energy, an electronic apparatus and a transportation device using the power generation unit, and a method of controlling the power generation unit.
2. Related Art
When a piezoelectric material such as lead zirconium titanate (PZT), quartz crystal (SiO2), or zinc oxide (ZnO) is deformed in response to an external force, electrical polarization is induced inside the material, and positive and negative charges appear on the surfaces. Such a phenomenon is called a so-called piezoelectric effect. There has been proposed an electrical power generation method of vibrating a cantilever to thereby make a weight repeatedly act on the piezoelectric material, and thus taking out the charge generated on the surface of the piezoelectric material as electricity using such a characteristic provided to the piezoelectric material.
For example, by vibrating a metal cantilever having a mass disposed at the tip and a thin plate made of the piezoelectric material bonded thereto, and taking out the positive and negative charges alternately generated on the piezoelectric material due to the vibration, an alternating current is generated. Then, the alternating current is rectified by diodes, and then stored in a capacitor, and then taken out as electricity. Such a technology has been proposed in JP-A-7-107752. Further, there has been also proposed a technology of arranging that a junction is closed only during the period in which the positive charges are generated in a piezoelectric device to thereby make it possible to obtain a direct current without causing a voltage loss in the diodes (JP-A-2005-312269). Since it is possible to miniaturize the power generation unit by using these technologies, there is expected an application of, for example, incorporating the power generation unit in, for example, a small-sized electronic component instead of a battery.
However, in the proposed technology according to the related art, there arises a problem that the obtainable voltage is limited up to the voltage generated by the electrical polarization of the piezoelectric material. Therefore, there arises a problem that a step-up circuit is required separately from the power generation circuit for taking out the electricity from the piezoelectric material in many cases, and it is difficult to sufficiently miniaturize the power generation unit.